Abstract
Hot springs and igneous rocks are present widely in southeast China, influenced by the subduction of the Western Pacific and Philippine Sea Plates. This study reports on new data of chemical compositions and He–Ne–C isotopes for gas samples from representative hot springs and wells in the Guangdong and Fujian provinces to identify the origin of hydrothermal volatiles and provide insight into geothermal tectonic affinities. The primary hydrothermal volatile component from southeast China is atmospheric N2, with a volumetric percentage of 82.19%–98.29%. It indicates medium-low temperature geothermal systems where geothermal fluids suffered a shallow circulation in closed fracture systems. Low CO2 and CH4 contents and their depleted δ13C values confirmed the small number of deep-derived components in the study area. However, spatially discernible geochemical characteristics imply enhanced hydrothermal fluid convection in the adjacent area of the two provinces, including the Fengshun, Zhangzhou, Longyan, and Sanming geothermal fields. Specifically, the He–Ne isotopes from this area exhibit mantle He contribution of more than 10% and mantle heat flow accounts for more than half of the total heat flow. Moreover, according to the thermal background calculations, the highest heat flow value of 77.7 mW/m2 is indicated for the Zhangzhou geothermal area and the lowest value of 54.7 mW/m2 is indicated for the Maoming geothermal area. Given the epicenter distributions and the corresponding earthquake magnitudes, the NE-trending faults are heat-control tectonic structures and their intersections with the NW-trending faults provided expedite channels for geothermal fluids rising to the surface. Therefore, the preferred development potential of geothermal resources can be expected in the adjacent area of the two provinces where two sets of active faults crossed. This study provides critical information on understanding the geothermal distribution controlled by the tectonic structure in southeast China.
Highlights
Convective geothermal systems, called fracture-controlled geothermal systems, are formed via deep-water circulation in tectonically active areas (Santilano et al, 2015)
We report chemical compositions and He–Ne–C isotopes for geothermal gas samples from representative hot springs and drilled wells in the Guangdong and Fujian provinces, southeast China, to identify the origin and evolution of hydrothermal volatiles
In granite-hosted southeast China, the active faults formed during the Neotectonic movement constrain the convection of mass and heat in geothermal fluids
Summary
Convective geothermal systems, called fracture-controlled geothermal systems, are formed via deep-water circulation in tectonically active areas (Santilano et al, 2015). Regional tectonic structures, where extensional channels formed by crossed faults provide channels for geothermal fluid rising to the surface (Jolie et al, 2015), control the formation of this type of system. This type of geothermal system is prominent in nonvolcanic areas with high-temperature backgrounds. In the eastern edge of the Qinghai–Tibet Plateau, the Xianshuihe fault, an active lithospheric-scale strike-slip fault, functions as a conduit for deep-derived geothermal volatiles, and enhances the heat convection process down to a depth of 8 km, forming hightemperature geothermal systems with reservoir temperatures as high as 260°C. Extensional deep faults in convective geothermal systems are typically the focus of geothermal exploration as preferential targets for high-temperature geothermal resources
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